Understanding life history variation in insects requires an assessment of how resources are allocated between reproductive and somatic traits. Associated data are not only a potential indicator of species adaptation, but also of environmental influences on fitness. So far, however, there is a dearth of data on factors affecting resource allocation, in particular in real environments. Using an integrative nutrient assay protocol, this study assessed the effects of body size and environmental conditions on resource allocation strategies in terrestrial and aquatic semelparous insects: the indian mealmoth, Plodia interpunctella under laboratory conditions, and two caddisfly species (Hydropsyche siltalai and Rhyacophila dorsalis) in the field. In Plodia interpunctella, larger females had higher potential lifetime fecundity due to their prolonged lifespan, while smaller females maintained greater initial egg loads and a shorter life-span. Assays revealed that current reproduction requires greater nutrient allocation in somatic and reproductive tissue, illustrating potentially fundamental factors underlying reproduction-longevity trade-offs. Field studies showed how trichopteran larvae have species-specific responses to physicochemical conditions located along an altitudinal gradient (20 m - 230 m asl) in the River Usk. Nutrient allocation in Rhyacophila dorsalis did not differ between upstream and downstream sites. In contrast, upstream Hydropsyche siltalai emerged with greater nutrient reserves consistent with early reproduction while those in warmer downstream environments must reallocate resources for somatic maintenance reducing available resource for reproduction. Consistent with this result, translocation of late instar H. siltalai over a thermal gradient (1.5 - 4.5 °C) showed how thermal exposure can affect adult fitness by reducing body size and increasing nutrient allocation for somatic maintenance. These results show how insects adjust nutrient balance for reproduction and survival under different circumstances. The most significant result is that global warming might reduce body size and advance reproduction in some species with consequences for fitness. This is proposed as an area ripe for further study to understand the evolutionary consequences of global change.